This invention relates to a method of cooling an electrolyte circulating through an electrolyzing section of a chemical plant used for the electrowinning or electrolytic recovery of zinc.
In the electrowinning of a metal, such as zinc, using a suitable electrolyte, the electrolyte is generally subjected to a temperature rise due to the heat generated during the electrolysis. Such a rise in the temperature of the electrolyte is especially undesirable for the successful electrowinning of zinc due to the fact that the lead anodes usually employed are subjected to corrosion and the metal electro deposited on the cathodes tends to be re-dissolved into the electrolyte as a result of attack by impurities existing in the electroylte, resulting in undesirable degradation of the quality of the product and undesirable reduction of the current efficiency. It is therefore essential to maintain a constant temperature of the electrolyte and at a desired low level in order to improve the efficiency of the electrolysis.
Various methods have hitherto been proposed for effectively cooling the electrolyte used for the electrowinning of zinc. These prior art methods include an indirect cooling method, a self-vaporizing method, a vacuum vaporizing method, and a direct cooling method. However, the first or indirect cooling method using a heat exchanger is defective in that a very large heat exchange area is required for effectively cooling the electrolyte by a cooling medium, and the cooling medium may leak into the electrolyte when the heat exchanger is corroded. This method is further defective in that the cooling efficiency is not so high.
The second or self-vaporizing method is defective in that a very large amount of cooling water is required for condensing the water vapor produced by vaporization.
The third or vaccum vaporizing method, using a vacuum pump, is defective in that troublesome maintenance often results from use of a vacuum pump. Further, this third method is not so effective for use in some geographical areas where the temperature of local cooling water is relatively high.
The fourth or direct cooling method using air for heat exchange is defective in that the efficiency of heat exchange is quite low and troublesome maintenance is required for the heat exchanger. Further, these prior art methods have the common defficiency that hydrates of impurities such as CaSO.sub.4, K.sub.2 SO.sub.4, MgSO.sub.4, Na.sub.2 SO.sub.4, MnSO.sub.4 and SiO.sub.2 existing in the electrolyte or neutral leaching solution tend to precipitate in gel form, and such impurities in gel form tend to deposit on the interior surfaces of the cooling apparatus. This deposition is objectionable in that not only the efficiency of heat exchange is lowered, but also, clogging of the conduits and other parts of the cooling apparatus result. Further, the impurities have a tendency to solidify and difficulty is encountered in removing the solids resulting from such solidification.